One node group input override another
You can do this by filtering on the Pink colour the Image Texture node outputs when no image is loaded. The only caveat is that it won't fully work if your Texture Image contains any pixels with RGB values of (1.0/0.0/1.0).



I've bundled the switching logic into a node for ease of viewing. Here's what the interior of the node group looks like:




Handling 2 Procedural Textures

In order to handle 1 or 2 procedural textures, you can set the default Colour on the Switch node to the Pink colour (R1/G0/B1). This way if nothing is connected it uses the default texture, and if the 2nd procedural texture is connected it uses that.



Blend file attached: